This invention relates to artificial magnetic conductors (AMCs) and devices incorporating AMCs. In particular, this invention relates to AMCs that are capable of operation at multiple separate frequency bands.
Due to the constant demand for improved efficiency of antennas and increased battery lifetime in portable communication systems, high-impedance surfaces have been the subject of increasing research. High-impedance surfaces have a number of properties that make them important for applications in communication equipment. The high-impedance surface is a lossless, reactive surface, whose equivalent surface impedance,       Z    s    =            E              t        ⁢                  xe2x80x83                ⁢        an                    H              t        ⁢                  xe2x80x83                ⁢        an            
(where Etan is the tangential electric field and Htan is tangential magnetic field), approximates an open circuit. The surface impedance inhibits the flow of equivalent tangential electric surface current and thereby approximates a zero tangential magnetic field, Htan≈0.
One of the main reasons that high-impedance surfaces are useful is because they offer boundary conditions that permit wire antennas (electric currents) to be well matched and to radiate efficiently when the wires are placed in very close proximity to this surface. Typically, antennas are disposed less than xcex/100 from the high-impedance surfaces (usually more like xcex/200), where xcex is the wavelength of operation. The radiation pattern from the antenna on a high-impedance surface is substantially confined to the upper half space, and the performance is unaffected even if the high-impedance surface is placed on top of another metal surface. The promise of an electrically-thin, efficient antenna is very appealing for countless wireless device and skin-embedded antenna applications.
One embodiment of a conventional frequency selective surface (FSS) 102 and AMC 100 is shown in FIG. 1. It is a printed circuit structure, using an electrically-thin, planar, periodic structure, with vertical conductors (vias) 104 forming a rodded medium, and horizontal capacitive patches 106, which can be fabricated using low cost printed circuit technologies. The combination of the FSS 102, connected to a ground plane 110 through a rodded medium is known as an artificial magnetic conductor (AMC). The rodded medium is periodic structure of parallel vertical conductors, or vias 104, embedded in a host dielectric medium that we denote as the spacer layer 108. Near its resonant frequency, the AMC approximates an open circuit to a normally incident plane wave, and it suppresses TE and TM surface waves over the band of frequencies near where it operates as a high-impedance surface.
An antenna, such as a bent-wire monopole, may be disposed within close proximity to the surface of the AMC, thus decreasing the overall thickness of the device. Bent-wire monopoles are primarily used as the antenna element that is integrated with an AMC. The bent-wire monopole is simply a thin wire or printed strip located a small fraction of a wavelength about xcex/200 above the AMC surface. The bent-wire monopole is disposed on the AMC surface using a thin layer of low loss dielectric material. Typically, a coaxial connector feeds one end of this strip antenna. The outer conductor of the coaxial connector is soldered to the conducting ground plane 110 of the AMC, and the inner conductor extends vertically through the AMC and a thin dielectric layer upon which the monopole is printed or disposed, to connect to the monopole.
Present communication applications, such as cellular telephones, may transmit and receive signals at several different frequency bands. The most popular of these frequency bands in North America are the GSM band (824-894 MHz) and the PCS band (1850-1990 MHz). In Europe, the GSM band covers 876-960 MHz, and the DCS band (1710-1880 MHz) is used. Conventional AMCs have only a single frequency band over which they exhibit high-impedance characteristics and surface waves are suppressed. Thus, applications requiring an antenna flush-mounted against a conventional AMC are limited to operation within the single frequency band. Multiple conventional AMCs/antenna combinations are needed to adequately operate within multiple frequency bands, thereby increasing the size and manufacturing cost of multi-frequency devices.
One object of the present invention is to provide a single, electrically-thin AMC that exhibits high-impedance characteristics and adequately suppresses surface waves in multiple frequency bands. Another object of the present invention is to decrease the size and cost of devices that incorporate AMCs and which operate in multiple non-harmonically related frequency bands.
In a first embodiment, a dual band AMC is modeled by an equivalent circuit having at least two shunt capacitors, which represent sheet capacitances of frequency selective surfaces of the AMC and at least two series inductive elements. The capacitors and inductive elements form an equivalent circuit which is a Cauer type I LC ladder network. The equivalent circuit has two non-harmonically related resonant frequencies.
The inductive elements may be electrically-short transmission lines or inductors. The resonant frequency bands may cover GSM and PCS, GSM and DCS, or GPS L1 and L2 bands, or other bands as dictated by the application.
Another embodiment is a method of establishing parameters (physical and/or frequency) of an AMC in at least desired two frequency bands. The AMC comprises at least two frequency selective surfaces, each having at least one layer of periodic conductive patches. The method comprises: choosing desired two frequency bands, choosing sheet capacitances for each FSS, selecting values for: gap widths between conductive patches of a first of the frequency selective surfaces, permittivities of dielectric layers between the layers that contain the conductive patches, thicknesses of the dielectric layers, and a chamfer distance for conductive patches of a second of the frequency selective surfaces. The method also comprises determining an overlap area of conductive patches on each FSS, determining a periodicity of the conductive patches on each FSS, and determining a chamfer distance for conductive patches of each FSS for which the chamfer distance was not selected.
An alternative embodiment of such a method comprises: grounding a first conductive layer, separating the frequency selective surfaces, separating the first conductive layer from one of the frequency selective surfaces, and connecting the periodic conductive patches on a first layer of the at least one layer of periodic conductive patches of a first frequency selective surface of the at least two frequency selective surfaces to the first conductive layer and connecting at least some of the periodic conductive patches on a first layer of the at least one layer of periodic conductive patches of a second frequency selective surface of the at least two frequency selective surfaces to the first conductive layer.
Another embodiment of a dual band AMC comprises a ground plane, a first FSS separated from the ground plane by a first dielectric layer and having a first layer and a second layer of overlapping periodic conductive patches separated by a second dielectric layer. The first layer of conductive patches is more proximate to the ground plane than the second layer of conductive patches. The AMC also comprises a second FSS separated from the first FSS by a third dielectric layer. The second FSS has a third layer of periodic conductive patches and is more distal to the ground plane than the first FSS.
The second FSS may further comprise a fourth layer of periodic conductive patches overlapping the third layer of conductive patches and separated from the third layer of conductive patches by a fourth dielectric layer. The fourth layer of conductive patches is more distal to the ground plane than the third layer of conductive patches.
Another FSS may further comprise a fifth layer of periodic conductive patches separated from the fourth layer of conductive patches by a fifth dielectric layer. The fifth layer of conductive patches is more distal to the ground plane than the fourth layer of conductive patches. The number of FSSs and layers of conductive patches on the FSS depends on the desired number and placement of non-harmonically related resonant frequencies.
Another embodiment of a dual band AMC comprises a ground plane, a plurality of FSS layers, each FSS layers having periodic conductive patches, a first dielectric layer separating each of the FSS layers, and a second dielectric layer separating the ground plane from one of the FSS layers.
In a yet another embodiment, a triple band AMC is modeled by an equivalent circuit having at least three shunt capacitors, which represent sheet capacitances of frequency selective surfaces of the AMC and at least three series inductive elements. Again, the capacitors and inductors form an equivalent circuit which is a Cauer type I network.